Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form a ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or a combination of beam scanning and target movement. Examples of prior art ion implanters are disclosed in U.S. Pat. No. 4,276,477 issued Jun. 30, 1981 to Enge; U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner; U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al; and U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. Device manufacturers need to critically control the depth distribution of implanted dopants. To achieve this, the ion implanter must critically control the energy of ions that impinge on the wafer surface. The requirement for energy control affects many requirements, such as power supply stability. However, the performance of ion implanters has been limited by less obvious causes of energy contamination, which is the presence in the ion beam of particles with energies that differ from the desired implant energy.
Energy contamination can result from interaction between ions in the beam and residual gas molecules in the system. Charge exchange reactions may change the charge state of beam ions when they interact with neutral molecules in the system. As might be expected, the probability of such an exchange occurring depends on the neutral gas density and therefore the system pressure. If, after such a reaction, the beam is accelerated by an electric field, then the ions that have changed charge state will, in the absence of further analysis, reach the target with the incorrect energy. This is because the energy gained by an ion in traversing an accelerating or decelerating electric field is proportional to the charge state of the ion.
The energy range of ion implanter is often extended by tuning the system to transport to the multiply charged ions that are produced by the source. In this way, for example, instead of using a 200 KV accelerator to implant 200 keV singly charged ions, 400 keV doubly charged ions can be implanted with suitable tuning. This approach, however, has problems due to the molecular ions that are produced by the source. Consider, for example, that the required ion on target is P.sup.++. Although the source may be tuned to maximize production of P.sup.++ ions, it will also generate other ions and in particular generates P.sub.2.sup.+ ions. This molecular ion is a well-known source of energy contamination, since it can break up to form P.sup.+ ions at almost exactly one-quarter of the energy of the required P.sup.++ ions. Magnetic analysis can not distinguish between P.sup.++ ions and P.sup.+ ions at one-quarter of the energy, and so ions at lower than required energy reach the target.
Along with ions of the required species, implanters often deposit contaminants onto the wafer surface. The contaminants may be in the form of particles or ions and molecules of another species. The contaminants can be produced by the ion source and transported through the beamline or, alternatively, may be generated by sputtering by energetic ions impinging on surfaces in the beamline.
Accordingly, there is a need for ion implanters wherein the ion beam that is implanted into the semiconductor wafer has low energy contamination and a low content of contaminants.